fix: ensure gas drip covers signed reads

[ameya-deshmukh]

Related to SEI-409 — https://linear.app/seismic-systems/issue/SEI-409/3-faucet-fund-mock-wallet-gas-reserves-with-susdc

Summary

• raise the documented sUSDC gas drip from 10,000 to 100,000 base units
• reject machine-funding configurations below 100,000 at startup
• cover the minimum with a configuration regression test

A signed sUSDC balance read on staging consumed 72,001 base units, so the previous amount required eight separate faucet calls before the E2E suite could start.

Test plan

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test --workspace --locked
• cargo build --workspace --release --locked

[linear-code]

SEI-409